Raised epitaxial LDD in MuGFETs

ABSTRACT

Embodiments include Multiple Gate Field-Effect Transistors (MuGFETs) and methods of forming them. In an embodiment, a structure includes a substrate, a fin, masking dielectric layer portions, and a raised epitaxial lightly doped source/drain (LDD) region. The substrate includes the fin. The masking dielectric layer portions are along sidewalls of the fin. An upper portion of the fin protrudes from the masking dielectric layer portions. A first spacer is along a sidewall of a gate structure over a channel region of the fin. A second spacer is along the first spacer. The raised epitaxial LDD region is on the upper portion of the fin, and the raised epitaxial LDD region adjoins a sidewall of the first spacer and is disposed under the second spacer. The raised epitaxial LDD region extends from the upper portion of the fin in at least two laterally opposed directions and a vertical direction.

BACKGROUND

Semiconductor devices are used in a large number of electronic devices, such as computers, cell phones, and others. Semiconductor devices comprise integrated circuits that are formed on semiconductor wafers by depositing many types of thin films of material over the semiconductor wafers, and patterning the thin films of material to form the integrated circuits. Integrated circuits typically include field-effect transistors (FETs).

Conventionally, planar FETs have been used in integrated circuits. However, with the ever increasing density and decreasing footprint requirements of modern semiconductor processing, planar FETs may generally incur problems when reduced in size. Some of these problems include sub-threshold swing degradation, significant drain induced barrier lowering (DIBL), fluctuation of device characteristics, and leakage. Multiple Gate FETs (MuGFETs), which include Fin FETs (finFETs), have been studied to overcome some of these problems.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1 through 5 illustrate a method of forming a fin and a gate structure according to an embodiment;

FIGS. 6A, 6B, 7A, 7B, 8A, 8B, 9A, 9B, 10A, 10B, 11A, 11B, and 11C are cross-sectional views of intermediate stages in the manufacturing of a MuGFET in accordance with an embodiment;

FIGS. 12A, 12B, and 12C are example structures used in manufacturing a MuGFET according to an embodiment; and

FIGS. 13A, 13B, and 13C are further example structures used in manufacturing a MuGFET according to an embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosed subject matter, and do not limit the scope of the different embodiments.

Multiple Gate Field-Effect Transistors (MuGFETs), which include Fin FETs (finFETs), and methods of forming the same are provided in accordance with various embodiments. The intermediate stages of forming the MuGFETs are illustrated. Some variations of the embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. Although method embodiments are discussed in a particular order, various other method embodiments may be performed in any logical order and may include fewer or more steps described herein. For example, some embodiments are discussed herein in the context of a gate-first process, whereas other embodiments contemplate a gate-last process.

FIGS. 1 through 5 illustrate an example method of forming a fin and a gate structure for a MuGFET. FIGS. 1 through 4 are cross sections along an X-Z plane, and FIG. 5 is a three dimensional view. FIG. 1 illustrates a substrate 20, which may be, e.g., a part of a wafer. Substrate 20 may be a semiconductor substrate, which may further be a silicon substrate, a silicon carbon substrate, a silicon germanium substrate, or a substrate formed of other semiconductor materials. The substrate 20 may be a bulk substrate, a semiconductor-on-insulator (SOI) substrate, or other acceptable substrates. The substrate 20 may be lightly doped with a p-type or an n-type impurity.

FIGS. 2 and 3 illustrate the formation of fins 22 and isolation regions 26 between the fins 22. In FIG. 2, the fins 22 are formed in the substrate 20, and a dielectric material 24 is formed between the fins 22. In some embodiments, the fins 22 may be formed in the substrate 20 by etching trenches in the substrate 20. The etching may be any acceptable etch process, such as a reactive ion etch (RIE), neutral beam etch (NBE), the like, or a combination thereof. The dielectric material 24 may be a high density plasma oxide, such as silicon oxide, formed by a high density plasma chemical vapor deposition (CVD) or a flowable CVD (FCVD), such as formed by a CVD-based material deposition in a remote plasma system and post curing to make it convert to oxide. Other dielectric materials formed by any acceptable process may be used. A planarization process, such as a chemical mechanical polish (CMP), may remove any excess dielectric material 24 and form top surfaces of the dielectric material 24 and top surfaces of the fins 22 that are co-planar.

In FIG. 3, the dielectric material 24 is recessed to form isolation regions 26, which may also be referred to as Shallow Trench Isolation (STI) regions. In FIG. 3, the isolation regions 26 are recessed such that fins 22 protrude from between neighboring isolation regions 26. The isolation regions 26 may be recessed using an acceptable etching process, such as one that is selective to the material of the isolation regions 26. For example, a chemical oxide removal using a Tokyo Electron CERTAS or an Applied Materials SICONI tool or dilute hydrofluoric acid may be used.

A person having ordinary skill in the art will readily understand that the process described with respect to FIGS. 2 and 3 is just one example of how fins 22 may be formed. In other embodiments, a dielectric layer can be formed over a top surface of the substrate 20; trenches can be etched through the dielectric layer; homo-epitaxial structures can be epitaxially grown in the trenches; and the dielectric layer can be recessed such that the homo-epitaxial structures protrude from the dielectric layer to form fins.

In still other embodiments, heteroepitaxial structures can be used for the fins. For example, the fins 22 in FIG. 2 can be recessed, and a material different from the fins 22 may be epitaxially grown in their place. In an even further embodiment, a dielectric layer can be formed over a top surface of the substrate 20; trenches can be etched through the dielectric layer; heteroepitaxial structures can be epitaxially grown in the trenches using a material different from the substrate 20; and the dielectric layer can be recessed such that the heteroepitaxial structures protrude from the dielectric layer to form fins.

In some embodiments where homo-epitaxial or heteroepitaxial structures are epitaxially grown, the grown materials may be in situ doped during growth, which may obviate some implantations although in situ and implantation doping may be used together. Still further, it may be advantageous to epitaxially grow a material in an NFET region different from the material in a PFET region. In various embodiments, the fins 22 may comprise or consist essentially of silicon, silicon germanium, silicon carbide, pure or substantially pure germanium, a III-V compound semiconductor, a II-VI compound semiconductor, or the like. For example, the available materials for forming III-V compound semiconductor include, but are not limited to, InAs, AlAs, GaAs, InP, GaN, InGaAs, InAlAs, GaSb, AlSb, AlP, GaP, and the like. A person of ordinary skill in the art will readily understand other methods of forming the fins, which are contemplated within the scope of some embodiments.

Although not depicted, a P well and/or an N well may be formed in various regions of the fins 22 and/or substrate 20. For example, during the processing discussed with respect to FIGS. 2 and 3, a photoresist can be formed over the fins 22 and the dielectric material 24 or the isolation regions 26. The photoresist can be patterned to expose a region of the substrate 20, such as an NFET region, by using a spin-on technique and acceptable photolithography techniques. Once patterned, a p-type impurity implant may be performed in the NFET region. The p-type impurities may be boron, BF₂, or the like implanted in the NFET region to a concentration of equal to or less than 7×10¹⁸ cm⁻³, such as between about 10¹⁵ cm⁻³ and about 7×10¹⁸ cm⁻³. After the implant, the photoresist may be removed, such as by an acceptable ashing process. Another photoresist may be patterned to expose another region of the substrate 20, such as a PFET region, using similar techniques, and an n-type impurity implant may be performed in the PFET region. The n-type impurities may be phosphorus, arsenic, or the like implanted in the PFET region to a concentration of equal to or less than 7×10¹⁸ cm⁻³, such as between about 10¹⁵ cm⁻³ and about 7×10¹⁸ cm⁻³. After the implant, the photoresist may be removed, such as by an acceptable ashing process. After the implants, an anneal may be performed to activate the p-type and n-type impurities that were implanted. The implantations may form a p-well in the NFET region and an n-well in the PFET region.

FIG. 4 illustrates a gate dielectric layer 28 formed over the fins 22 and the isolation regions 26, a gate electrode layer 30 over the gate dielectric layer 28, and a mask layer 32 over the gate electrode layer 30. In some embodiments, gate dielectric layer 28 comprises silicon oxide, silicon nitride, or multilayers thereof. In other embodiments, gate dielectric layer 28 comprises a high-k dielectric material, and in these embodiments, gate dielectric layer 28 may have a k value greater than about 7.0, and may include a metal oxide or a silicate of Hf, Al, Zr, La, Mg, Ba, Ti, Pb, and combinations thereof. The formation methods of gate dielectric layer 28 may include Molecular-Beam Deposition (MBD), Atomic Layer Deposition (ALD), Plasma Enhanced Chemical Vapor Deposition (PECVD), thermal oxidation, and the like. The gate electrode layer 30 may comprise polysilicon (doped or undoped), a metal-containing material such as TiN, TaN, TaC, Co, Ru, Al, combinations thereof, or multi-layers thereof. The gate electrode layer 30 may be deposited using CVD, ALD, physical vapor deposition (PVD), the like, or a combination thereof. The mask layer 32 may comprise silicon nitride, silicon oxide, silicon oxynitride, the like, or a combination thereof, and may be deposited using CVD, ALD, PVD, the like, or a combination thereof.

FIG. 5 illustrates an example of an intermediate MuGFET in a three-dimensional view. In FIG. 5, the mask layer 32, the gate electrode layer 30, and the gate dielectric layer 28 are patterned to form a mask 44, a gate electrode 42, and a gate dielectric 40, respectively. The patterning may use an acceptable photolithography and etch process(es), such as an RIE, CCP, ICP, the like, or a combination thereof. The etch may be selective to a desired material and may be anisotropic. The gate dielectric layer 28 may be removed from top surfaces of the fins 22, but remaining masking portions 46 of the gate dielectric layer 28 may remain on sidewalls of the fin 22 due to the etch being anisotropic. The remaining masking portions 46 may be exposed on the side of the fin 22. The remaining masking portions 46 on the sidewalls of the fin 22 may expose an upper portion of the fin 22, such as to a distance of approximately 2 nm to 10 nm from the top surface of the fin 22. This upper portion exposure may result from the etch that patterns the gate electrode 42 and/or gate dielectric 40, or by a subsequent etch.

FIG. 5 further illustrates various cross sections used for reference in other figures. Cross section A-A is in a Y-Z plane and is illustrated as a reference for FIGS. 6A, 7A, 8A, 9A, 10A, and 11A. Cross section B-B is in an X-Z plane and is illustrated as a reference for FIGS. 6B, 7B, 8B, 9B, 10B, and 11B. Further, cross section B-B may be a reference plane for the cross sections in FIGS. 1 through 4, although any X-Z plane may be illustrated in those figures.

FIGS. 6A and 6B illustrate respective cross sections of the intermediate MuGFET of FIG. 5. FIG. 6A illustrates the substrate 20 including the fin 22, a gate dielectric 40 over a channel region of the fin 22, a gate electrode 42 over the gate dielectric 40, and the mask 44 over the gate electrode 42. FIG. 6B illustrates the substrate 20 including the fin 22, the isolation regions 26 adjacent the fin 22, and the remaining masking portions 46 on sidewalls of the fin 22.

In FIGS. 7A and 7B, an offset spacer layer 48 is conformally deposited over the intermediate MuGFET structure in FIGS. 5, 6A, and 6B. The offset spacer layer 48 may be silicon nitride (SiN), silicon carbon-nitride (SiCN), silicon carbon-oxynitride (SiCON), the like, or a combination thereof, formed by CVD, ALD, the like, or a combination thereof.

In FIGS. 8A and 8B, the offset spacer layer 48 is anisotropically etched to form offset spacers 50 on sidewalls of the gate structure, e.g., at least adjoining the gate dielectric 40 and gate electrode 42. Also as a result of this etch, remaining offset portions 52 remain on sidewalls of the fin 22. The etch may be an RIE, CCP, ICP, transformer coupled plasma (TCP), a high density plasma (HDP) etch, electron cyclotron resonance (ECR) etch, the like, or a combination thereof. The remaining offset portions 52 on the sidewalls of the fin 22 may expose an upper portion of the fin 22, for example, the etch may remove a portion of the offset spacer layer 48 from a distance of approximately 2 nm to 10 nm from the top surface of the fin 22.

In FIGS. 9A and 9B, a raised epitaxial layer 54 is epitaxially grown on exposed surfaces of the fin 22, and the raised epitaxial layer 54 and portions of the fin 22 are implanted with dopants to form lightly doped source/drain (LDD) regions 56, which may also be referred to as source/drain extension regions. The raised epitaxial layer 54 may comprise or consist essentially of silicon, silicon germanium, germanium, silicon carbide, indium phosphide (InP), gallium arsenide (GaAs), the like, or a combination thereof. The epitaxial growth may be performed using metal-organic CVD (MOCVD), molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), selective epitaxial growth (SEG), the like, or a combination thereof. The remaining masking portions 46, remaining offset portions 52, or a combination thereof may mask lower portions of the fin 22 to prevent epitaxial growth from occurring on those lower portions of the fins 22. A thickness T (see for example, FIGS. 12C and 13C) of the epitaxial layer 54 measured in a direction perpendicular from a corresponding surface of the fin 22 may be between approximately 2 nm and 10 nm, although the thickness may vary depending on the technology node, for example. The LDD regions 56 may be implanted to a dopant concentration of equal to or greater than 10¹⁹ cm⁻³. The dopants may include, for example, boron, indium, or the like for a p-type transistor, and may include, for example, phosphorus, arsenic, or the like for an n-type transistor.

In FIGS. 10A and 10B, main spacers 58 are formed along sidewalls of the offset spacers 50 and over portions of the epitaxial layer 54. The main spacers 58 may be formed by depositing a main spacer layer and anisotropically etching the main spacer layer. The main spacer layer may be silicon nitride (SiN), silicon carbon-nitride (SiCN), silicon carbon-oxynitride (SiCON), the like, or a combination thereof, formed by CVD, ALD, MOCVD, the like, or a combination thereof. The etch may be an RIE, CCP, ICP, TCP, HDP etch, ECR, the like, or a combination thereof.

In FIGS. 11A, 11B, and 11C, the fin 22 is etched to recess source/drain areas of the fin 22, and raised epitaxial source/drain regions 62 are epitaxially grown. The etch may be selective to exposed portions of the raised epitaxial layer 54 and the fin 22, and the remaining masking portions 46, remaining offset portions 52, the mask 44, offset spacers 50, and main spacers 58 may act as a mask to prevent etching in other portions of the fin 22. The raised epitaxial source/drain regions 62 may comprise or consist essentially of silicon, silicon germanium, germanium, silicon carbide, indium phosphide (InP), gallium arsenide (GaAs), the like, or a combination thereof, and may be epitaxially grown using MOCVD, MBE, LPE, VPE, SEG, the like, or a combination thereof. The raised epitaxial source/drain regions 62 extend or protrude from an original surface of the fin 22. The raised epitaxial source/drain regions 62 may further be doped to a concentration of greater than or equal to 5×10¹⁹ cm⁻³ by in situ doping during growth and/or subsequent implantation. The dopants may include, for example, boron, indium, or the like for a p-type transistor, and may include, for example, phosphorus, arsenic, or the like for an n-type transistor.

As illustrated in FIGS. 11A, 11B, and 11C, the etch to form the raised epitaxial source/drain regions 62 removes exposed portions of the raised epitaxial layer 54. Hence, as illustrated, the portions of the raised epitaxial layer 54 under the main spacers 58 remain and form raised epitaxial LDD regions 60. FIG. 11B, which shows the cross section B-B, illustrates the raised epitaxial LDD region 60 under the main spacer 58, and FIG. 11C, which illustrates the cross section C-C, shows the raised epitaxial source/drain regions 62 where the raised epitaxial layer 54 was removed.

The crystalline structure upon which raised epitaxial layer 54 is grown may affect the shape of the raised epitaxial layer 54, and hence, the raised epitaxial LDD regions 60. FIGS. 12A-C and 13A-C illustrate some example conditions. FIG. 12A illustrates a substrate 70, e.g., a wafer having a (001) Si surface, and a notch 78 in the <110> direction. A fin 72 is on the substrate 70. In FIG. 12B, the fin 72 comprises a (001) Si top surface 80, a (110) Si sidewall surface 82, and a (110) Si end surface 84. A gate structure 74, such as including gate dielectric 40, gate electrode 42, and mask 44 is over a channel region of the fin 72. With these surfaces, a faceted raised epitaxial layer 76 with (111) surfaces may be grown on the fin 72. This may result because of the starting epitaxy crystalline surface orientation. FIG. 13A illustrates a substrate 90, e.g., a wafer having a (001) Si surface, and a notch 98 in the <100> direction. A fin 92 is on the substrate 90. In FIG. 13B, the fin 92 comprises a (001) Si top surface 100, a (010) Si sidewall surface 102, and a (100) Si end surface 104. A gate structure 94, such as including gate dielectric 40, gate electrode 42, and mask 44 is over a channel region of the fin 92. With these surfaces, a conformal raised epitaxial layer 96 may be grown on the fin 92. This may result because of the starting epitaxy crystalline surface orientation. FIGS. 12C and 13C further illustrate example thicknesses T, which may be between approximately 2 nm and 10 nm, although other thicknesses may be used, especially for differing technology nodes.

Embodiments may achieve advantages. As shown in the figures, some embodiments may employ a raised epitaxial LDD region on an upper portion of the fin. This raised epitaxial LDD region is grown in three directions, e.g., two opposite lateral directions from the fin and a vertical direction from the fin. This shape may reduce parasitic resistance in a MuGFET by reducing a current density through the LDD region. Further, since only the upper portion of the fin has the raised epitaxial LDD region and not on all of the fin sidewall, short channel effect advantages may remain in the MuGFET. The raised epitaxial LDD region may also enhance carrier mobility through the MuGFET.

An embodiment is a structure comprising a substrate, a fin, masking dielectric layer portions, a gate structure, a first spacer, a second spacer, a raised epitaxial lightly doped source/drain (LDD) region, and a source/drain region. The substrate includes the fin and isolation regions adjoining the fin. The masking dielectric layer portions are along sidewalls of the fin and extend from the isolation regions. An upper portion of the fin protrudes from the masking dielectric layer portions. The gate structure is over a channel region of the fin. The first spacer is along a sidewall of the gate structure. The second spacer is along the first spacer, and the first spacer is disposed between the second spacer and the gate structure. The raised epitaxial LDD region is on the upper portion of the fin, and the raised epitaxial LDD region adjoins a sidewall of the first spacer and is disposed under the second spacer. The raised epitaxial LDD region extends from the upper portion of the fin in at least two laterally opposed directions and a vertical direction. The source/drain region is in the fin.

Another embodiment is a structure comprising a semiconductor fin on a substrate, a gate structure, a first spacer, a second spacer, a raised epitaxial lightly doped source/drain (LDD) region, a mask layer, and a source/drain region. The gate structure is along and over a channel region in the semiconductor fin. The first spacer adjoins a sidewall of the gate structure. The second spacer adjoins the first spacer, and the first spacer is disposed between the gate structure and the second spacer. The raised epitaxial LDD region is on the semiconductor fin. The raised epitaxial LDD region laterally adjoins the first spacer and is disposed between the second spacer and the semiconductor fin. The raise epitaxial LDD region extends from the semiconductor fin in at least three distinct directions orthogonal to respective surfaces of the semiconductor fin. The mask layer adjoins the raised epitaxial LDD region and extends from the raised epitaxial LDD region adjoining a sidewall of the semiconductor fin to an isolation region. The source/drain region in the semiconductor fin is proximate the raised epitaxial LDD region.

A further embodiment is a method comprising forming a fin on a substrate, isolation region portions being proximate first and second sidewalls of the fin, the first and second sidewalls being on opposing sides of the fin; forming a gate structure over the fin and along a channel region of the fin; forming a first spacer adjoining a sidewall of the gate structure; forming masking layer portions adjoining respective lower portions of the first and second sidewalls of the fin and extending from respective isolation region portions, the mask layer portions exposing upper portions of the first and second sidewalls of the fin and an upper surface of the fin; epitaxially growing a raised lightly doped source/drain (LDD) layer on the upper portions of the first and second sidewalls of the fin and the upper surface of the fin, the epitaxially growing using the mask layer to prevent epitaxial growth on the lower portions of the first and second sidewalls of the fin; forming a second spacer adjoining the first spacer and over the raised LDD layer; and forming a source/drain region in the fin.

Although the present embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A structure comprising: a substrate comprising a fin and isolation regions adjoining the fin; a masking dielectric layer portions along sidewalls of the fin and extending from the isolation regions, an upper portion of the fin protruding from the masking dielectric layer portions; a gate structure over a channel region of the fin; a first spacer along a sidewall of the gate structure; a second spacer along the first spacer, the first spacer being disposed between the second spacer and the gate structure; a raised epitaxial lightly doped source/drain (LDD) region on the upper portion of the fin, the raised epitaxial LDD region adjoining a sidewall of the first spacer and being disposed under the second spacer, the raised epitaxial LDD region extending from the upper portion of the fin in at least two laterally opposed directions and a vertical direction; and a source/drain region in the fin.
 2. The structure of claim 1, wherein the raised epitaxial LDD region comprises a facet with respect to an underlying surface of the fin.
 3. The structure of claim 1, wherein the raised epitaxial LDD region is conformal to an underlying surface of the fin.
 4. The structure of claim 1, wherein the substrate is a (001) Si substrate, and the sidewalls of the fin are (110) Si surfaces.
 5. The structure of claim 1, wherein the substrate is a (001) Si substrate, and the sidewalls of the fin are (010) Si surfaces.
 6. The structure of claim 1, wherein the source/drain region comprises a raised epitaxial source/drain region in a recess of the fin and extending beyond a surface of the fin.
 7. A structure comprising: a semiconductor fin on a substrate; a gate structure along and over a channel region in the semiconductor fin; a first spacer adjoining a sidewall of the gate structure; a second spacer adjoining the first spacer, the first spacer being disposed between the gate structure and the second spacer; a raised epitaxial lightly doped source/drain (LDD) region on the semiconductor fin, the raised epitaxial LDD region laterally adjoining the first spacer and being disposed between the second spacer and the semiconductor fin, the raise epitaxial LDD region extending from the semiconductor fin in at least three distinct directions orthogonal to respective surfaces of the semiconductor fin; a mask layer adjoining the raised epitaxial LDD region and extending from the raised epitaxial LDD region adjoining a sidewall of the semiconductor fin to an isolation region; and a source/drain region in the semiconductor fin proximate the raised epitaxial LDD region.
 8. The structure of claim 7, wherein the raised epitaxial LDD region comprises a (111) facet.
 9. The structure of claim 7, wherein the raised epitaxial LDD region is conformal to the semiconductor fin.
 10. The structure of claim 7, wherein the substrate is a (001) Si substrate, and the sidewall of the semiconductor fin is a (110) Si surface.
 11. The structure of claim 7, wherein the substrate is a (001) Si substrate, and the sidewall of the semiconductor fin is a (010) Si surface.
 12. The structure of claim 7, wherein the source/drain region comprises a raised epitaxial source/drain region in a recess of the semiconductor fin and extending beyond a surface of the semiconductor fin.
 13. A method comprising: a semiconductor fin on a substrate, the semiconductor fin having a first sidewall, a second sidewalls, and an upper surface between the first sidewall and the second sidewall; a gate dielectric and gate electrode along the first sidewall, the upper surface, and the second sidewall; a first dielectric spacer contacting a sidewall of the gate electrode; a second dielectric spacer contacting the first dielectric spacer; a raised epitaxial lightly doped source/drain (LDD) region on the first sidewall, the upper surface, and the second sidewall, the raised epitaxial LDD region contacting the first dielectric spacer and being disposed between the second dielectric spacer and the semiconductor fin; a mask layer extending from the raised epitaxial LDD region along the first sidewall and the second sidewall to respective isolation regions; and a source/drain region in the semiconductor fin proximate the raised epitaxial LDD region.
 14. The structure of claim 13, wherein the raised epitaxial LDD region comprises a (111) facet.
 15. The structure of claim 13, wherein the raised epitaxial LDD region is conformal to the first sidewall, the upper surface, and the second sidewall.
 16. The structure of claim 13, wherein the substrate is a (001) Si substrate, and at least one of the first sidewall and the second sidewall is a (110) Si surface.
 17. The structure of claim 13, wherein the substrate is a (001) Si substrate, and at least one of the first sidewall and second sidewall is a (010) Si surface.
 18. The structure of claim 13, wherein the source/drain region comprises an epitaxial source/drain region disposed between the mask layer along the first. sidewall and the second sidewall and extending above the beyond the mask layer, the epitaxial source/drain region contacting the raised epitaxial LDD region.
 19. The structure of claim 13, wherein the mask layer comprises a first sub-layer and a second sub-layer, the first sub-layer contacting the first sidewall and the second sidewall, the first sub-layer being disposed between th second sub-layer and the semiconductor fin.
 20. The method of claim 19, wherein the first sub-layer and the gate dielectric are portions of a same dielectric layer. 